Welcome to this forum with your first post
In order for you to get rid of the starting Logo such as "DIGITAL" go to BIOS and disable the fast start, then mach sure your hard drive in recognized in IDE Auto Detect, then save the setting and reboot, if you still get the 'OPERATING SYSTEM NOT FOUND' message then there could be two reason.
1- Operating system does not exist
2- Operating system exist by the drive is not set to "active mode"
I hope that helps
Cheers